High performance microelectronic devices often use solder balls or solder bumps for electrical interconnection to other microelectronic devices. For example, a very large scale integration (VLSI) chip may be electrically connected to a circuit board or other next level packaging substrate using solder balls or solder bumps. This connection technology is also referred to as "Controlled Collapse Chip Connection--C4" or "flip-chip" technology, and will be referred to herein as "solder bumps".
In the original solder bump technology developed by IBM, the solder bumps are formed by evaporation through openings in a shadow mask which is clamped to an integrated circuit wafer. Solder bump technology based on a electroplating process has also been actively pursued, particularly for larger substrates and smaller bumps. In this process, an "under-bump metallurgy" (UBM) is deposited on a microelectronic substrate having contact pads thereon, typically by evaporation or sputtering. A continuous under-bump metallurgy film is typically provided on the pads and on the substrate between the pads, in order to allow current flow during solder plating.
In order to define the sites for solder bump formation over the contact pads, the sites of the solder bumps are photolithographically patterned, by depositing a thick layer of photoresist on the under-bump metallurgy and patterning the photoresist to expose the under-bump metallurgy over the contact pads. Solder pads are then formed on the exposed areas of the under-bump metallurgy, over the contact pads, by pattern electroplating. The plated solder accumulates in the cavities of the photoresist, over the contact pads. Then, the under-bump metallurgy between the plated solder pads is etched, using the solder as an etch mask, to break the electrical connection between the solder bumps. The photolithographic patterning and under-bump metallurgy etching steps define the geometry of the under-bump metallurgy at the base of the solder bump, between the solder bump and the contact pad. Solder bump fabrication methods are described in U.S. Pat. Nos. 4,950,623 to Dishon, assigned to the assignee of the present invention; 4,940,181 to Juskey, Jr. et al.; and 4,763,829 to Sherry.
Unfortunately, in fabricating solder bumps using the process described above, it is difficult to preserve the base of the solder bump, at the contact pad. Preservation of the base is important because the base of the solder bump is designed to seal the contact pad. The process described above often reduces the base, which exposes the underlying contact pad and leads to mechanical and/or electrical failure.
The base may be reduced due to at least two steps in the above described process. First, there is often an inherent distortion of the patterned thick film photoresist layer, and misalignment with respect to the contact pads lying thereunder. Typically, a dry thick film photoresist (such as du Pont RISTON.RTM. photoresist) or multiple coatings of liquid photoresist is used, in order to accumulate sufficient volume of plated solder. Thicknesses on the order of tens of microns (for example 50 microns) are used. The thick film photoresist must be accurately patterned over the contact pads, without misalignment or distortion.
Unfortunately, for dry film photoresist, distortion of the shape of bump sites may result from the relatively poor adhesion of the photoresist to the smooth under-bump metallurgy. Light scattering through the thick film photoresist and cover layer, and the imprecise nature of the thick film photoresist development process, also contribute to distortion of the photoresist mask pattern over the contact pads. For multiple-layer liquid photoresist, factors such as hardening of photoresist due to long periods of baking, and edge bead build-up, may cause distortion in the photoresist mask pattern over the contact pads. Accordingly, the resultant solder bump often does not cover the entire contact pad.
The second major factor which may reduce the solder bump base is undercutting during chemical etching of the under-bump metallurgy. In particular, as described above, the under-bump metallurgy is typically etched, between the solder bumps, in order to break the electrical connections therebetween. In order to insure that all of the unwanted under-bump metallurgy is removed, overetching typically needs to be practiced, because etching frequently does not proceed uniformly across the substrate surface. However, this overetching typically undercuts the under-bump metallurgy between the solder bump and the contact pad, which reduces the solder bump base. Electrical and mechanical reliability of the solder bump connection is thereby degraded.